1. Field of the Invention
This invention relates to electric power-generating devices such as wind and ocean current turbines, and more particularly to a method and apparatus of control of the stator voltage on induction generators to allow control of the induction generator speed.
2. Description of the Prior Art
Induction generators have certain advantages over synchronous generators. Synchronous generators require separate field windings on the generator rotor which are excited by a separate DC voltage source. This makes a synchronous generator rotor complicated, asymmetrical and difficult to operate at high speeds. Induction generators do not have field windings, resulting in a more symmetrical structure. Therefore, induction generators are cheaper to manufacture and more rugged than synchronous generators. In addition, the rotor of the induction generator can “slip,” which means to operate at speed of rotation greater than the AC mains frequency. If the generators stator voltage is not reduced in some manner, however, the slip is limited to a small change, (2 to 5) %, of the synchronous speed.
The current in the rotor of an induction generator is induced by the rotating stator magnetic field whenever the rotor is not at synchronous speed. The maximum speed difference or “slip” is limited by the amount of rotor current, which will cause overheating of the rotor.
It is desirable to provide an induction generator free of the above drawbacks, so as to produce an induction generator which is practical to use.
It is also desirable to provide an induction generator which can be operated easily over a wide range of speeds, with a high level of stability and with the ability to easily control such mechanical quantities as torque and speed. When the stator voltage is controllably reduced, greater speed increases are possible, up to 15%, which permits a wind turbine rotor blade to speed up during a wind gust. Speeding up in this way is very desirable, as it limits the maximum loads on the wind turbine.